I've come with this solution:

------------------------------------------------------------------------

diff --git a/drivers/mfd/axp20x.c b/drivers/mfd/axp20x.c
index 012c064..117eacb 100644
--- a/drivers/mfd/axp20x.c
+++ b/drivers/mfd/axp20x.c

@@ -882,7 +882,7 @@ EXPORT_SYMBOL(axp20x_match_device);

 int axp20x_device_probe(struct axp20x_dev *axp20x)
 {
-	int ret;
+	int ret, irq_base;

 	ret = regmap_add_irq_chip(axp20x->regmap, axp20x->irq,
 				  IRQF_ONESHOT | IRQF_SHARED, -1,

@@ -893,8 +893,9 @@ int axp20x_device_probe(struct axp20x_dev *axp20x)
 		return ret;
 	}

+	irq_base = regmap_irq_chip_get_base(axp20x->regmap_irqc);
 	ret = mfd_add_devices(axp20x->dev, -1, axp20x->cells,
-			      axp20x->nr_cells, NULL, 0, NULL);
+			      axp20x->nr_cells, NULL, irq_base, NULL);

 	if (ret) {
 		dev_err(axp20x->dev, "failed to add MFD devices: %d\n", ret);
------------------------------------------------------------------------

However, this implies that all cells added by the mfd driver which are
requesting irqs will need to be changed in the same commit to remove the
regmap_irq_get_virq calls. If we don't modify the drivers, they will
purely fail to request the irqs.

The impacted drivers are the following:

 - drivers/extcon/extcon-axp288.c
 - drivers/input/misc/axp20x-pek.c
 - drivers/power/supply/axp20x_usb_power.c
 - drivers/power/supply/axp288_charger.c
 - drivers/power/supply/axp288_fuel_gauge.c

Is it really worth to do such a cleanup?